It is a discovery of the invention that the hydrolysis of hemicellulose in cellulosic materials such as hard woods, straw, and other plant material is biphasic, i.e., that in the case of hardwoods, about 70% of the hemicellulose can be hydrolyzed much faster (fast hydrolyzable xylan) than the remaining 30% (slow hydrolyzable xylan).